Is There a Place for the TRH Stimulation Test?

A Rare Test That Is Not Available in the United States

Lab technician with blood samples and medical chart
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There is more than one test that can be used to evaluate how your thyroid is functioning.

Besides testing the thyroid stimulating hormone (TSH) or thyroxine level (T4), a doctor may use the thyrotropin-releasing hormone (TRH) stimulation test. That said, this test is rarely used; in fact, it's not available in the United States, although it's available in other countries.

 

Understanding the TRH Test

To understand how the TRH stimulation test works, it's first helpful to quickly review how the various brain hormones interact with the thyroid:

  • Your hypothalamus is a gland in your brain that secretes thyrotropin-releasing hormone (TRH).
  • When TRH is released, it stimulates your brain's pituitary gland to release thyroid stimulating hormone (TSH).
  • TSH stimulates the thyroid itself to make thyroid hormones.

Most doctors use the TSH test, which measures the circulating level of TSH in the bloodstream at one point in time. The TSH test results are then interpreted; if your levels are higher or lower than the reference range, this is considered evidence of potential thyroid disease.

The TRH test is different. First, a baseline TSH test is done. Then you're given a dose of TRH through your vein which stimulates your pituitary to release TSH. A second blood sample is then drawn 20 to 60 minutes later, and the TSH level is retested.

Some practitioners feel that the TRH test can detect subtle thyroid problems and measure your thyroid's ability to respond in real time, compared to the TSH test, which is a snapshot of thyroid function at one point in time.

How the TRH Test Differs From the TSH Test

Comparing the TRH stimulation test and the TSH test is much like a cardiac stress test compared to an electrocardiogram, or a glucose tolerance test compared to fasting glucose level.

In a stimulation test, the challenge may reveal an impairment in the thyroid.

Still, most doctors consider the TSH test highly accurate with only one blood draw required and no special supplies. Moreover, the TSH is inexpensive. In comparison, the labor-intensive and more costly TRH test requires two separate draws a half hour apart, the availability of TRH, and the knowledge of how to accurately perform and interpret the test.

Benefits of the TRH Test

The TRH test is occasionally used to help identify secondary hypothyroidism (hypothyroidism that's due to pituitary problems) and tertiary hypothyroidism (hypothyroidism that's due to hypothalamic disorder).

However, while the TRH can be used to detect secondary or tertiary hypothyroidism (collectively called central hypothyroidism), it really cannot be used to distinguish between pituitary versus hypothalamic disease.

Conclusion

Ultimately, researchers have shown that the TRH stimulation test may be used to detect central hypothyroidism from diseases not related to the thyroid. 

Earlier research also suggests that the TRH stimulation test may reveal what's referred to as early sub-biochemical hypothyroidism before it's reflected in the more common TSH test.

Perhaps, new studies will be conducted to determine the potential benefits to patients of more widespread use of this test.

Sources:

Doi SAR, Issac D, Abalkhail S, Al-Qudhaiby MM, Hafez MF, Al-Shoumer KAS. TRH Stimulation When Basal TSH is Within the Normal Range: Is There “Sub-Biochemical” Hypothyroidism? Clinical Medicine & Research. 2007;5(3):145-148. doi:10.3121/cmr.2007.756.

Ross DS. (2017). Central hypothyroidism. Cooper DS, ed. UpToDate. Waltham, MA: UpToDate Inc.